1. Field of the Invention
The present invention relates to a thermal head comprising heating resistors, particularly to a thermal head having high printing efficiency and reliability.
2. Description of the Prior Art
Thermal heads are widely used for various heat-sensitive recording systems. A thermal head comprises a substrate and a plurality of heating resistors formed on the substrate and constituting printing elements or dots, and is designed so that the printing elements can be heated in any optional combination by applying an electrical current selectively thereto.
The heating resistors of the thermal heads which have been commonly employed, are made of metals, oxides or other compounds, such as TaN, Ta-Si, Ta-SiO or Cr-SiO. However, in most of these heating resistors, the temperature coefficient of resistance (TCR) tends to decrease at a high temperature, and in many cases, if an electric power is applied excessively, reckless overheating takes place at a high temperature, thus leading to the destruction of the heating resistors. Further, when an electric current is applied to a heating resistor, there is a general tendency that the central portion is heated to a higher temperature than the peripheral portion since the peripheral portion releases the heat more quickly than the central portion. If the temperature coefficient of resistance decreases at a high temperature, as mentioned above, the temperature at the center portion becomes even higher, and the temperature distribution on the surface of the heating resistor becomes uneven, whereby the useful life of the heating resistor will be shortened, and the printing efficiency will be poor. As a technique to overcome the above drawbacks, it has been proposed that a slender strip of a heating resistor is formed in a zigzag fashion so that every portion has the same surface density. However, the surface area of a printing element (a dot) is presently about 100.times.200 .mu.m, and in order to form a strip of about 30 .mu.m, an etching technique with high precision is required. Further, with this method, it will be very difficult to realize a high resolving power such as 16 dots/mm.sup.2 in a prospective future.
Under the circumstances, the present inventors considered that in order to avoid such difficulties, it was necessary to improve the material for the heating resistors, and have conducted extensive researches to reach the present invention.